This invention relates to polyester film or sheet that is produced by calendering, and more particularly, to the calendered polyester film or sheet that has a haze value of less than 5 percent.
Calendering is an economic and highly efficient means to produce film and sheet from plastics. Polyvinyl chloride (PVC) resins are by far the largest segment of the calendered film and sheet business, although small amounts of other thermoplastic polymers such as polypropylene, acrylonitrile/butadiene/styrene terpolymers, and chlorinated polyethylene are also processed by calendering methods.
U.S. Pat. No. 6,068,910 discloses that amorphous polyesters and copolyester compositions with crystallization half-times from a molten state of at least 5 minutes may be calendered if an additive for preventing sticking to the calendering rolls is blended with the resin. Examples of such additives include fatty acid amides, metal salts of organic acids, fatty acids and esters, hydrocarbon waxes, polyethylene waxes, chemically modified polyolefin waxes, and ester waxes.
U.S. Pat. No. 5,998,005 also discloses that particular copolyester resins can be calendered if a fatty acid ester lubricant such as stearic acid monoglyceride, a montanic acid ester, or a montanic acid ester partially saponified with calcium, is added to the resin. The particular copolyester resin is obtained by replacing in the production of polyethylene terephthalate (PET) resin 10 to 40 mole % of the ethylene glycol component with 1,4-cyclohexanedimethanol. The invention also teaches that the copolyester resins may comprise, in addition to the copolyester and fatty acid ester lubricant, another lubricant such as a fatty acid, a fatty acid amide, a fatty acid bisamide, or a fatty acid ketone.
Calenderable compositions of amorphous PET using phosphate esters and/or fatty acid ester release agents are disclosed in Japan Unexamined Patent Publication 2000-186,191A. The haze of calendered sheet made from such compositions was measured to be less than 40% according to JIS K 7105 Standard Test Method for Haze.
U.S. Pat. Nos. 6,068,910 and 5,998,005 do not report quantitative values for haze in the calendered polyester sheets. U.S. Pat. No. 5,998,005 does disclose that transparency is good with the preferred release agent. In many applications involving film and sheet of amorphous polyesters, 40% haze and even good clarity are insufficient.
Thus, there exists a need in the art for a polyester composition that when utilized in the calendering process exhibits good release from the calendering rolls and displays excellent clarity in the final sheet or film product. Accordingly, it is to the provision of such that the present invention is primarily directed.
A calendered film or sheet of a polyester resin composition has a haze value of less than 5 percent. The polyester resin composition comprises a polyester having a crystallization half-time from a molten state of at least 5 minutes and an additive comprising (i) a fatty acid or a salt of a fatty acid containing more than 18 carbon atoms and (ii) an ester wax comprising a fatty acid containing more than 18 carbon atoms with an alcohol containing from 2 to 28 carbon atoms, wherein the ratio of the fatty acid or salt of the fatty acid to the ester wax is 1:1 or greater. Also, the present invention is a process for producing such calendered film or sheet having a haze value of less than 5 percent. The steps of the process comprise (1) calendering such polyester resin composition, (2) forming on a calendering roll a film or sheet from such polyester resin composition, and (3) removing the film or sheet from the calendering roll at an angle of less than 90xc2x0 to form a film or sheet having a haze value of less than 5 percent.